This invention relates to a color cathode ray tube incorporating means for supporting an aperture mask therein, and more particularly relates to such a tube incorporating an improved thermal compensation type of mask supporting means for higher Q-spacing.
Most color cathode ray tubes (CCRTs) employ "aperture masks", that is, color selection electrodes of formcd thin metal sheets having a multitude of apertures positioned to allow each of the three electron beams to strike only on set of the three primary (red, blue and green) color-emitting phosphor elements on the screen. The thin aperture mask is supported within the tube by a relatively massive and rigid frame, which in turn is supported in the tube by the engagement of frame brackets with mounting studs protruding from the sidewall of the glass viewing panel.
Most of the electron beams emitted from the electron gun never reach the phosphor elements on the screen, but instead are intercepted by the aperture mask and the electron shield between the mask frame and the panel sidewalls. It is estimated that only about 15% of the electron beams actually are transmitted through the mask to the screen. Thus, a large amount of electron beam energy is dissipated as heat within the tube
The apertured portion of the mask has a contoured surface of roughly spherical shape, corresponding in general to the screen contour on the interior surface of the viewing panel. The space between the aperture mask and the screen is commonly referred to as the "Q-space". Maintaining this Q-space during tube operation is critical in order to maintain registration between the scanning electron beams and the phosphor elements on the screen.
During initial warmup, the heat generated by the electron beams striking the aperture mask causes the mask to expand laterally due to its relatively low mass, high thermal conductivity and high thermal expansion coefficient. This expansion tends to displace the aperture in a direction away from the center of the mask and toward the edges of the mask. After a short time the mask frame also expands bringing the components toward equilibrium. However, because the apertures are now displaced further from the center of the mask, the Q-space must be reduced in order to restore registration between the mask apertures and the phosphor elements on the screen.
Bimetallic element have been incorporated into the mask frame support means to cause the mask frame assembly to move toward the screen during initial tube warmup, thus reducing Q-space to compensate for mask expansion, and maintaining registration during tube operation. See, for example, U.S. Pat. No. 3,524,973.
A successful application of bimetal design to obtain thermal compensation utilizes two relatively narrow elongated dissimilar metal elements joined edge to edge along their length dimensions to form an elongated bimetal spring bracket. One end of this bracket is fixed to the mask frame sidewall, and the other end is apertured to engage mounting studs protruding from the glass panel sidewall. One of the elements is L-shaped to provide a tab at the free end to accommodate the aperture. The aperture is in the shape of an equi-lateral triangle with rounded apexes, and is oriented with one base normal to the bimetal joint.
These bimetal spring brackets are formed to extend outwardly from the frame sidewall toward the panel sidewall. This outward spring bias of the brackets necessitates flexing them inwardly during insertion of the mask-frame assembly into the panel, and serves to maintain firm contact with the panel studs during tube operation.
Recent trends toward compaction of the color cathode ray tube and its components have led to the "mini-neck" design, in which the inline electron gun is miniaturized to fit into a rube neck which has been reduced to about 22 millimeters in diameter from 29 millimeters (narrow neck) and 36 millimeters (standard neck) diameters. Such design results in savings both in material and in the power needed to deflect the electron beams during TV operation.
Unfortunately, the design also results in a significantly larger Q-space, due to the closer spacing of the three inline electron beams in the gun. One way of achieving such increased Q-space is to locate the panel studs farther back from the screen on the panel sidewall. However, this approach requires creation of a non-standard panel type, complicating manufacture and reducing the ability to recycle scrap. Another way of increasing Q-space is to weld the brackets closer to the screen side of the frame sidewall. However, in current designs, this would require removal of a portion of the mask sidewall or skirt which telescopes over the outside of the frame sidewall, and is needed to maintain adequate mask strength.
Accordingly, it is an object of the present invention to provide a color cathode ray tube with an improved mask support means that will enable achievement of the Q-space required for compact tubes such as "mini-neck" tubes, while also providing the desired temperature compensation for such tubes. It is also an object of the invention to achieve increased Q-space in color cathode ray tubes without moving the mounting studs in the face panel, and without removing any portion of the mask skirt. It is also an object of the invention to achieve increased Q-space with minimum change to the color cathode ray tube.